(Sub)Antarctic Coastal Marine Sediments
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Research Collection Journal Article CompoundSpecific Radiocarbon Analysis of (Sub)Antarctic Coastal Marine Sediments—Potential and Challenges for Chronologies Author(s): Berg, Sonja; Jivcov, Sandra; Kusch, Stephanie; Kuhn, Gerhard; Wacker, Lukas; Rethemeyer, Janet Publication Date: 2020-10 Permanent Link: https://doi.org/10.3929/ethz-b-000448962 Originally published in: Paleoceanography and Paleoclimatology 35(10), http://doi.org/10.1029/2020PA003890 Rights / License: Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library RESEARCH ARTICLE Compound‐Specific Radiocarbon Analysis of (Sub‐) 10.1029/2020PA003890 Antarctic Coastal Marine Sediments—Potential Key Points: • Phytoplankton‐derived low and Challenges for Chronologies molecular weight fatty acids can be S. Berg1 , S. Jivcov1 , S. Kusch2 , G. Kuhn3 , L. Wacker4 , and J. Rethemeyer1 used to date Antarctic marine sediments 1Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany, 2Department of Prehistoric Archaeology, • Contribution of petrogenic, 3 reworked carbon in Antarctic and University of Cologne, Cologne, Germany, Alfred Wegener Institute (AWI), Helmholtz Centre for Polar and Marine Subantarctic shelf settings can be Research, Bremerhaven, Germany, 4Ion Beam Physics, ETH Zurich, Switzerland assessed by compound‐specific radiocarbon analysis • Land plant‐derived high molecular Abstract In Antarctic and Subantarctic environments, 14C‐based age determination is often challenging weight fatty acids in the Subantarctic are systematically due to unknown reservoir effects, low organic carbon contents of sediments, and high contributions of 14 depleted in radiocarbon petrogenic ( C‐free) carbon in ice marginal settings. In this study, we evaluate possible benefits and challenges of compound‐specific radiocarbon analysis (CSRA) as a tool for age determination of marine Supporting Information: Antarctic and Subantarctic sediment sequences. We present a comprehensive data set of 14C ages obtained • Supporting Information S1 on bulk organic carbon, carbonates, and on fatty acids (FA) from three coastal marine sediment cores from Subantarctic South Georgia and East Antarctica. Low molecular weight (LMW) FA represent the least 14 ‐ ‐ Correspondence to: C depleted fraction, indicating that the phytoplankton derived compounds can be a means of dating 14 S. Berg, sediments. In contrast, vascular plant‐derived high molecular weight FA are systematically depleted in C [email protected] relative to the low molecular weight homologues, reflecting processes such as soil formation/erosion in the catchment. Comparative age‐depth models show significant differences, depending on the material used Citation: for the respective models. While the land plant‐derived FA may lead to an overestimation of the actual Berg, S., Jivcov, S., Kusch, S., Kuhn, G., sediment age, LMW FA reveal complex aquatic reservoir effects. Bulk sedimentary organic carbon 14C ages Wacker, L., & Rethemeyer, J. (2020). Compound‐specific radiocarbon likely provide appropriate age estimates in settings with low petrogenic carbon input in the Antarctic, analysis of (sub‐)Antarctic coastal whereas CSRA has the potential to produce improved age control in settings with high contributions of marine sediments—Potential and petrogenic carbon. challenges for chronologies. Paleoceanography and Paleoclimatology, 35, e2020PA003890. https://doi.org/10.1029/2020PA003890 1. Introduction Received 20 FEB 2020 Accepted 15 SEP 2020 Radiocarbon dating is the most widely used method for age determination of sedimentary records for the 14 Accepted article online 24 SEP 2020 Late Pleistocene and Holocene period (<55,000 years). In general, the C isotopic composition of organic matter (OM) and carbonates reflects the 14C isotopic composition of the primary carbon pool utilized by the respective organisms. While terrestrial OM mostly forms in equilibrium with atmospheric 14C, reservoir effects often impede a simple interpretation of sedimentary 14C ages in aquatic environments (marine and lacustrine, e.g., Gordon & Harkness, 1992; Hendy & Hall, 2006; Moreton et al., 2004). In the Southern Ocean, sea ice cover and more importantly regional upwelling of deep‐water masses lead to reservoir ages of on average 1,100–1,300 years (e.g., Berkman & Forman, 1996). Surface water reservoir ages may change regionally over time due to changes in oceanography and freshwater influx from the continent (e.g., Hall et al., 2010; van Beek et al., 2002). Estimation of reservoir ages is particularly difficult in isolated basins and marine inlets in coastal areas, which are influenced by changing marine and freshwater contributions. However, records from these areas are important for relative sea level reconstructions and provide informa- tion on climate and glaciation histories (e.g., Berg, White, Jivcov, et al., 2019; Hodgson et al., 2016). In marine sediments, carbonate fossils like foraminifer tests are frequently used for reconstructing sediment ©2020. The Authors. ages (e.g., Graham et al., 2017). However, across the Southern Ocean, carbonates are poorly or not at all pre- This is an open access article under the served in the sediments and 14C analysis of bulk sedimentary organic carbon (OC) is often used for age deter- terms of the Creative Commons 14 Attribution‐NonCommercial‐NoDerivs mination (e.g., Hillenbrand et al., 2009). In surface sediments of the Southern Ocean, C ages of bulk OC License, which permits use and distri- often exceed the marine reservoir age reflecting the admixture of petrogenic or reworked OC, especially in bution in any medium, provided the settings close to ice sheets and glaciers and/or when autochthonous primary production is low or organic original work is properly cited, the use is non‐commercial and no modifica- matter is not well preserved (e.g., Andrews et al., 1999; Ohkouchi & Eglinton, 2006; Subt et al., 2017). In tions or adaptations are made. coastal areas of the Subantarctic, organic matter derived from plants, soils, and peat deposits are BERG ET AL. 1of16 Paleoceanography and Paleoclimatology 10.1029/2020PA003890 (a) (b) Figure 1. (a) Marine coring locations Co1305 (Berg, White, Jivcov, et al., 2019), and PS81/283‐1 (Bohrmann, 2013) in Cumberland West Bay, South Georgia. Dashed lines indicate outlet glaciers terminating in Cumberland Bay. (b) Rauer Group, an ice‐free coastal archipelago in East Antarctica. Numbers indicate sites of previous studies applying compound‐specific and ramped pyrolysis 14C analyses in Antarctic sediments: 1 Yamane et al. (2014); 2 Yokoyama et al. (2016), Ohkouchi and Eglinton (2008); 3 Subt et al. (2017); 4 Rosenheim et al. (2008); 5 Rosenheim et al. (2013). additional OC sources (Berg, White, Jivcov, et al., 2019). Several studies showed that terrestrial OC deposited in marine sediments is often considerably 14C‐depleted upon deposition, which reflects the input of material stored for decades to thousands of years on land prior to transport into the ocean (e.g., Drenzek et al., 2007; Kusch et al., 2010; Schefuß et al., 2016; Vonk et al., 2014; Winterfeld et al., 2018). The 14C ages of terrestrial OC are controlled by many variables such as climatic conditions, vegetation and soil type, morphology and geology of the drainage areas (e.g., Cui et al., 2017; Drenzek et al., 2007; Kusch et al., 2010; Vonk et al., 2014). Evidence for these processes primarily comes from temperate regions or the Arctic, but thus far no studies investigate the degree of 14C depletion of terrestrial OM in marine sediments in Subantarctic marine environments. Different 14C techniques have been developed to separate 14C signatures of OC from different sources. For example, ramped pyrolysis (RP) employs sequential heating to release OC fractions from sediments based on their thermochemical stability allowing for the quantification of refractory and labile OC (e.g., Rosenheim et al., 2008, 2013). Compound‐specific 14C analysis (CSRA) separates individual organic mole- cules from sediments using preparative gas chromatography (PCGC) or high performance liquid chromato- graphy (e.g., Eglinton et al., 1996; Ingalls & Pearson, 2005). For example, in the Ross Sea, Antarctica, CSRA of phytoplankton‐derived low molecular weight (LMW) fatty acids (containing 14, 16, and 18 C atoms) in surface sediments has been shown to match the dissolved inorganic carbon (DIC) 14C isotopic composition of the water column (e.g., Ohkouchi & Eglinton, 2008), while substantial contributions of petrogenic OC in sediments restrict meaningful age determination using bulk OC 14C analysis (e.g., Andrews et al., 1999). This makes sedimentary LMW fatty acids (FA) primary targets for 14C analysis to provide age information when carbonate microfossils are absent and contributions of petrogenic OC are suspected. A recent study by Yokoyama et al. (2016) presented first core chronologies of Ross Sea sediments based on CSRA of LMW FA, which aided a better understanding of ice shelf instability during the Holocene (Figure 1). However, despite the great potential of such data until now only few studies have used CSRA to improve age‐depth models for Antarctic and Subantarctic marine sediments (Berg, White, Jivcov, et al., 2019; Ohkouchi & Eglinton, 2008; Yamane et al., 2014; Yokoyama et al., 2016, Figure 1). In this study, we evaluate possible benefits and challenges of CSRA as